This invention relates to an auto-ranging system for an electronic electrical energy meter.
In the past the metering of electrical energy, such as that flowing into homes, has been performed by electrical mechanical devices which are somewhat cumbersome because of the size thereof and in addition relatively expensive to maintain because of the moving mechanical parts thereof. More recently, as disclosed in the patent application of Miran Milkovic, Ser. No. 361,030, filed May 17, 1973 now U.S. Pat. No. 3,875,509, and in a second application of Miran Milkovic, Ser. No. 395,142, filed Sept. 7, 1973 now U.S. Pat. No. 3,875,508, solid state electronic meters are disclosed for measuring the flow of electrical energy in a power system. These applications disclose a method and apparatus for detecting the current flow through at least one line and the voltage of the line which parameters are appropriately multiplied and converted to a pulse train signal which is counted in a display mechanism to provide an indication of the electrical energy being consumed. These solid state electrical energy meters, however, have a critical drawback in that the dynamic range thereof is somewhat limited and the signal-to-offset or signal-to-noise ratio must be improved in order to provide a more accurate meter for measuring a wide range of power flow past a given point.
In the past, efforts have been made to improve the dynamic range and signal-to-noise ratio of solid state electrical energy meters as exemplified by the auto-ranging device proposed by Friedl and Seyfried in September 1972 in London at an IEE International Conference on Metering and Tariff. The device proposed by Friedl et al. utilizes two current transformers, one for supplying a voltage proportional to the current detected to a multiplier which multiplies the current proportional voltage by the voltage detected across the secondary of a potential transformer in the power system. The output of the multiplier is converted to a pulse train and is then coupled to an output display mechanism via a frequency divider. A second current transformer provides a current level signal to a range detector which in turn calibrates the level of the current proportional voltage so that as the current level increases past each of a plurality of discrete levels, the voltage which is proportional to the current decreases incrementally. In order to provide an overall system transfer function which is constant, that is, to maintain the ratio of pulses at the output of the system to the power input constant, the range detector coupled to the second transformer provides a signal for varying the frequency divider in discrete steps so that as the current input increases past each of the discrete levels, the dividing factor of the frequency divider decreases by a factor sufficient to maintain the transfer function of the solid state meter constant over a wide range. The Friedl et al. compensation device requires a second current transformer which increases the cost of the solid state electrical energy meter and in addition increases the cost of installing the meter in a locality. Further, a frequency divider is required which further adds to the expense of providing a relatively economical yet accurate metering system.
It therefore is an object of this invention to provide an accurate and reliable solid state electrical energy metering device having a wide dynamic range.
It is another object of this invention to provide an economical and reliable electrical energy meter having a relatively high signal-to-noise ratio and having a wide dynamic range.